I have just replaced EL34s in my SSE with KT88s. I am using 620R for cathode resistors with ~450v B+. I have some 2k resistors I can parallell for ~470R but other than slight decrease in distortion and slight increase in damping factor, would there be any benefit to increasing the current or can I just stick with the 620R?

That pretty much sums it up. Running them hotter improves their linearity. I would try both and use whichever sounds the best to your ears. That is all that matters in the end. Your KT88s should last a very long time at the lower current, so if you can't tell the difference then just do that.

use whichever sounds the best to your ears. That is all that matters in the end.

Yeah, what he says.....

On my big speakers I can hear a noticible difference when I put the 1K resistor across the 560 ohm that is in my board. This makes the tubes hot and the power transformer even hotter. On my other speakers it makes no difference at all, so why stress the parts?

I have a pair of resistors with aligator clips that I use when getting loud with the big speakers. They are not used any other time. Bench measurements show minimal differences and even a few tenths of a watt loss in power at the higher current since the B+ drops a bit.

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Too much power is almost enough! Turn it up till it explodes - then back up just a little.